1. Field of the Invention
The invention relates generally to the field of photographic cameras and in particular to those cameras in which a filmstrip is first prewound from a cartridge onto a take-up spool without exposing any of the frames on the filmstrip and then is rewound one frame at a time back into the cartridge after each exposure is completed.
2. Description of the Prior Art
Typically in a number of 35 mm cameras, there is included a motorized film transport mechanism. In operation, a leading end portion of the filmstrip extending from a light-tight cartridge loaded in the camera is attached automatically to a take-up spool. The take-up spool is rotated after each exposure to advance successive frames of the filmstrip from the cartridge across the focal plane of a taking lens and onto the take-up spool. Each time the take-up spool is rotated, an unexposed frame on the filmstrip is located in the focal plane of the taking lens and an exposed frame is wound onto the take-up spool. At the same time, a frame counter is incremented by rotation of a metering sprocket in engagement with successive perforations in the filmstrip to provide a count of the number of frames exposed or remaining to be exposed on the filmstrip. When the fresh film supply has been exhausted, a trailing end portion of the filmstrip remains attached to a supply spool within the cartridge. This end of film condition is signalled by a sudden rise in the film tension as the take-up spool attempts to withdraw the remainder of the filmstrip from the cartridge. A tension sensing mechanism responds to the increase in film tension by reversing the motor drive to rotate the supply spool within the cartridge. The rotated spool draws the exposed filmstrip off the take-up spool and rewinds it into the cartridge. Then, a rear door of the camera is opened and the cartridge is removed in order to process the filmstrip.
Opening the rear door of the camera, for example, to remove a cartridge containing an exposed filmstrip, usually initializes the frame counter to an original empty setting. When a cartridge with a fresh filmstrip is loaded in the camera and the door is re-closed, the frame counter is incremented from its empty setting to a "1" setting in response to advance of the filmstrip to locate the first frame in the focal plane of the taking lens. Failure to initialize the frame counter to its empty setting before the cartridge with the fresh filmstrip is loaded in the camera would result in the frame count being out of synchronization with the exposed frames on the filmstrip.
If the rear door of the camera is accidentally or inadvertently opened before the filmstrip is completely wound off the take-up spool and back into the cartridge, the exposed frames on the take-up spool may be ruined by stray light. To prevent this occurance, cameras have been recently provided with a motorized film transport mechanism which prewinds substantially the entire length of the filmstrip onto the take-up spool before any exposures are taken. Then, after each exposure, the filmstrip is rewound one frame at a time back into the cartridge. Thus, the exposed frames will be protected within the cartridge from stray light should the rear door be opened prematurely.
The frame counter in a camera that first prewinds a fresh filmstrip onto the take-up spool and then rewinds the filmstrip incrementally into the cartridge after each exposure is driven by rotation of the metering sprocket during film movement in both the prewind and rewind directions. When the filmstrip is moved in the prewind direction, the frame counter is incremented to indicate the maximum number of available exposures. Conversely, movement of the filmstrip in the rewind direction decrements the frame counter once after each exposure to indicate the remaining number of available exposures. Opening the rear door of the camera accidentally or inadvertently before the filmstrip is completely wound off the take-up spool usually initializes the counter to its empty setting. This, presumably, serves as a remainder to the photographer to press a button or to otherwise activate the motorized film transport mechanism to rewind the remainder of the filmstrip back into the cartridge.
It has been suggested in U.S. Pat. No. 4,460,256, granted July 17, 1984, that the filmstrip remaining on the take-up spool in an expose on rewind type camera be automatically rewound into the cartridge should the rear door of the camera be opened prematurely. In this instance, the frame counter is not initialized to its empty setting when the door is opened, but instead remains at its numbered setting above "0". Respective switches in the camera operate to sense that the door is open and that the frame counter is at a numbered setting above "0". This combination of events sensed by the switches triggers energization of a film drive motor in a rewind direction. Then, as the filmstrip is rewound into the cartridge, the frame counter is decremented in accordance with rotation of the metering sprocket. When the frame counter is decremented to a "0" setting, the corresponding switch changes state to de-energize the film drive motor. While this arrangement may perform satisfactorily, it is possible that the filmstrip will slip off the metering sprocket during rewind because the rear door is open and, therefore, the filmstrip is not held in engagement with the metering sprocket. As a result, the frame counter may be decremented to its "0" setting to de-energize the drive motor before the filmstrip to completely would off the take-up spool. This is due to the fact in the U.S. Pat. No. 4,460,256 that during rewind the metering sprocket is motor driven to decrement the frame counter at a faster speed than the supply spool within the cartridge is motor driven to wind the filmstrip onto the supply spool. Thus, the photographer may have to unwind some of the filmstrip off the take-up spool and hand feed it into the cartridge.